For downscaling and high integration of a semiconductor storage device such as a NAND flash memory, the width of word lines (control gates) or the interval between adjacent ones of the word lines has been narrowed. When the interval between adjacent ones of the word lines is narrowed, a dielectric breakdown or a leak current may occur between the adjacent ones of the word lines. To suppress the dielectric breakdown or the leak current, provision of an air gap (a hollow) between adjacent ones of the word lines is considered. Because a relative permittivity of the air gap (that is, air) is lower than that of an insulating film such as a silicon dioxide film, the air gap between adjacent ones of the word lines can reduce a parasitic capacitance between the adjacent ones of the word lines and enhance the withstanding pressure between the adjacent ones of the word lines.
However, when an insulating film is formed on the word lines, the insulating film is shrunk by heat treatment or the like and pulls the word lines to apply a tensile stress to the word lines. Therefore, when an air gap is provided between adjacent ones of the word lines, the word lines are curved due to the tensile stress. When the word lines are curved, charge retention properties of memory cells corresponding to the word lines are deteriorated.
For example, when an air gap is provided between adjacent ones of memory cells in a memory string, memory cells on both ends of the memory string are pulled by an insulating film on the memory string toward the center of the memory string. In this case, charge retention properties of the memory cells on the both ends of the memory string are deteriorated.